New Delhi, Oct. 20 -- Can you remember the last time a shopkeeper handed you a toffee from a jar because they did not have small change? For decades, this was the norm at kirana stores and pharmacies across India. Today, that jar has all but disappeared. Some say an unintended consequence of the UPI revolution has been a secular decline in candy sales. That is partly because, over the past ten years, UPI has rewritten the payment experience for customers and merchants alike.

The controversy over the return of the Merchant Discount Rate, or MDR, on UPI payments frames it as a consequential pricing question. But before arguing over who should pay for digital payments, it is worth revisiting how arduous person-to-merchant (P2M) payments onc...